Although Giardia lamblia cysts transmit the disease, virtually nothing is known of their antigenic composition, or of how the trophozoite form that colonizes the human small intestine differentiates into the resistant cyst. We found that exposing trophozoites to specific intestinal factors induces both expression of many neo-antigens and encystation, which we will continue to analyze on the molecular level. Therefore, we propose the following: Specific Aim 1: Our partial sequence of Gene 3.3, which was selected with antiserum against cyst walls, shows three areas of very strong homology with thiolases, which catalyze pathways thought not to be present in Giardia. We will now: (a) complete the sequencing of the gene and upstream regions to confirm the homology and look for regulatory regions; (b) assess its regulation during encystation; (c) assay thiolase-like activity in Giardia extracts; (d) prepare antibodies against the recombinant protein to identify the Giardia protein and localize it immunocytochemically. Specific Aim 2: Clone 2.1, which was selected with anticyst antibodies, reacts with only one band in southern blots, but in Northern blots recognizes three mRNA species whose relative intensities change during encystation. Therefore, we will (a) clone and sequence the rest of the gene; (b) use probes for different portions of the gene to map the three mRNAs and elucidate their regulation; (c) identify and characterize its protein product(s). Specific Aim 3: To determine whether protein expression, glycosylation, or both are regulated and to clone the protein, we will isolate, deglycosylate, and make antibodies against the protein moiety of the pH- regulated, Con A binding glycoproteins that are expressed during encystation. Specific Aim 4: We will test the hypothesis that giardiasis may be controlled in part by alphabeta or gammadelta T cells in the gut epithelium of mice by infecting SCID mice and mice with rearranged alphabeta or gammadelta T cell receptors. We will also ask whether cytokines produced locally may affect G lamblia growth or encystation. These studies will provide new insights into molecular and biochemical control mechanisms, as well as potential defenses against an important intestinal pathogen.